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Full text of "Text Book Of Mechanical Engineering"

Appendix VI.

1059

metal to extrude at the rail-head, which must be chipped away
when quite cold, and sometimes, for this reason, the drawing up
is not performed; but it is certainly advisable, for a true weld of
87% the strength of the solid rail is thereby produced even
without the extra band of iron. The latter, however, serves as a
most efficient fishplate of a material very similar to mild steel, and
is most thoroughly amalgamated with the rail metal. The usual
tram rail requires 22 Ibs. of thermit (at ivd. per Ib.) to each
weld, containing about n Ibs. of pure iron. To this is generally
added 15% by weight of iron punchings, thus yielding 14 Ibs. of
iron to the 22 of the mixture, a procedure not only more
economical, but also a means of reducing the extremely high
temperature within manageable limits. The cost of the process
compares favourably with electric welding.

There are three ways of using thermit practically, (i) The
iron may be run before the slag, as already described, by tapping
the crucible at the bottom, causing the highest possible tempera-
ture. (2) The crucible may be tapped from the top, running the
slag first, which, meeting the parts to be welded, coats them with
a resistant film. This prevents the close contact of the molten
iron, and reduces the temperature to a reasonable welding heat
between the surfaces of union, the joint being upset by clamps.
This method is adopted for iron or steel pipes, and gives a
perfectly clean joint after the removal of slag. (3) The rolls of
rolling mills often break off at the ends, and are to be patched
with an entirely new piece. The broken roll being sunk in the
ground, end up, a mould is made from a pattern and placed on
the top. The bottom of the mould (or end of roll) is covered to
| in. depth with molten cast iron, and the whole then filled with
thermit at the rate of 30 Ibs. to the square foot of joining surface,
then ignited in the usual manner. In this way cast iron and steel
can be connected, and the surfaces become as one. Section bars
of all kinds may be welded by the first or second methods as pre-
ferred ; while if the third method be adopted for any purpose
where molten steel is the material .required, the heat of the latter
will be sufficient without the necessity of external ignition. Steel
examples would be the stern frames of ships, which have been
welded and patched very successfully.